1. Field of the Invention
The present invention relates to a liquid crystal display device used in a direct viewing type or projection type display apparatus.
2. Description of Related Art
FIG. 1A is a partial sectional view of a device showing an arrangement of a conventional reflection type liquid crystal display device. The liquid crystal display device has a basic structure in which two substrates each having electrodes formed on one surface are stuck to each other with a predetermined gap such that the electrodes are opposite to each other, and a liquid crystal is injected into the gap. FIG. 1A shows an arrangement of a MOS type active matrix liquid crystal display device in which transistors are arranged for pixels, respectively. A portion corresponding to one pixel is typically shown in FIG. 1A.
As shown in FIG. 1A, in many reflection type liquid crystal display devices, a transparent glass substrate 210 is used as one of two stuck substrates, and a silicon substrate 290 is used as the other substrate. A transparent electrode 220 consisting of ITO (Indium Tin Oxide) or the like is formed on the inner side of the glass substrate 210, and an alignment film 230a for regulating the array of liquid crystal molecules is formed on the surface of the transparent electrode 220.
A transistor 270 or, depending on cases, as shown in FIG. 1A, a capacitor 280 is formed on the inner surface layer of the silicon substrate 290, and an Al reflective electrode 250 functioning as an electrode and a light reflecting film is formed on these elements through an insulating interlayer 260. An alignment film 230b is also formed on the surface of the Al reflective electrode 250.
A predetermined gap is kept between the glass substrate 210 and the silicon substrate 290 by a spacer (not shown), and a liquid crystal layer 240 is formed in the gap by injection.
In such a reflection type liquid crystal display device, light I0 from a light source is admitted from the glass substrate 210 side, passes through the liquid crystal layer 240, and is reflected by the Al reflective electrode 250 on the surface of the silicon substrate 290. When the light I0 of incidence and reflected light IR pass through the liquid crystal layer, polarization directions are regulated depending on the alignment state of the liquid crystal molecules.
In general, in a transmission type liquid crystal display device, a region in which a transistor is formed serves as a light-shielding portion and cannot transmit light, and it is difficult to structurally obtain a high aperture ratio. However, in a reflection type liquid crystal display device, a transistor can be formed on the lower layer of the reflective electrode 250. For this reason, the reflection type liquid crystal display device is more advantageous than the transmission type liquid crystal display device with respect to an aperture ratio.
FIG. 2 is a diagram simply showing an arrangement of a conventional projection type television set, i.e., a so-called liquid crystal projector, using the reflection type liquid crystal display device described above. As shown in FIG. 2, the liquid crystal projector has, as main components, a light source 100, two polarizers 120a and 120b, a half mirror 130, a liquid crystal display device 140, a drive circuit 150 for a liquid crystal display device, an optical lens 160, and a screen 170.
For example, light emitted from the light source 100 passes through the polarizer 120a, and only a polarized light component of a predetermined direction is extracted from the light. Thereafter, the light component is changed by the half mirror 130 in a course, and is admitted on the liquid crystal display device 140. On the liquid crystal layer in the liquid crystal display device 140, a predetermined voltage is applied to each pixel through the drive circuit 150, and, accordingly, the alignment state of the liquid crystal molecules is changed depending on the predetermined voltage. The polarization direction of a light component transmitted through the liquid crystal layer is regulated by the alignment state of the liquid crystal molecules. The light component reflected by the reflective electrode surface of the liquid crystal display device passes through the half mirror 130 to reach the other polarizer 120b. Only a polarized light component of a predetermined direction is selected by the polarizer 120b, and the polarized light component is enlarged through the optical lens 160 to be projected on the screen 170.
FIG. 1B is a partial sectional view of a device obtained by extracting a portion around the liquid crystal layer 240 in the liquid crystal display device shown in FIG. 1A. As shown in FIG. 1B, a part of light I0 being incident on the glass substrate 210 is reflected by each layer interface in the middle of the way to the liquid crystal layer 240. In particular, since ITO or tin oxide (SnO2) serving as a transparent electrode has a high refractive index of about 2, reflection easily occurs on the interface between the transparent electrode and the glass substrate or the alignment film. For example, the light I0 is admitted on the liquid crystal layer 240, reflected light components r11, r21, and r31 (These light components are referred to as an interface reflected light component R1 for descriptive convenience hereinafter.) are generated by the interface between the glass substrate 210 and the transparent electrode 220, the interface between the transparent electrode 220 and the alignment film 230a, and the interface between the alignment film 230a and the liquid crystal layer 240, respectively.
When the reflected light IR reflected by the reflective electrode 250 is admitted, reflected light components r32, r22, and r12 (These light components are referred to as interface reflected light R2 for descriptive convenience hereinafter) are generated by the interfaces of the respective layers, respectively. The interface reflected light R2 is reflected by the reflective electrode 250 again.
Since the interface reflected light R1 does not pass through the liquid crystal layer 240 and is not changed in a polarization direction, in many cases, the interface reflected light R1 is cut by the two polarizer 120b before reaching the screen and rarely influence the screen (see FIG. 2). However, the interface reflected light R2 which passes through the liquid crystal layer 240 once and is reflected by the reflective electrode 250 to be generated is rarely cut by the polarizer 120b, and the interface reflected light R2 reaches the screen together with the reflected light IR.
FIG. 3 is a graph showing a result obtained by measuring the reflectance of light corresponding to the interface reflected light R2 generated in the arrangement of the conventional liquid crystal display device shown in FIG. 1B. Comparative Example 1 in FIG. 3 is a liquid crystal display device in which an ITO film having a thickness of 400 xc3x85 and formed as the transparent electrode 220 and an SiO2 film having a thickness of 25 xc3x85 and formed as the alignment films 230a and 230b are used. Alignment properties are given to the alignment film such that inclined vapor deposition is performed to a substrate which is inclined at 70xc2x0 with respect to a vapor deposition source. Comparative Example 2 in FIG. 3 is a liquid crystal display device in which an ITO film having a thickness of 400 xc3x85 and formed as the transparent electrode 220 and a vertical alignment polyimide film having a thickness of 700 xc3x85 and formed as the alignment films 230a and 230b are used. The vertical alignment polyimide film is applied by a printing method and thermally hardened. The surface of the polyimide film is rubbed to give alignment properties to the polyimide film.
As shown in FIG. 3, the interface reflected light R2 reflected by the reflective electrode 250 and generated by the interface between the liquid crystal layer and a glass exhibits a reflectance of about 3% to 7% in a visible region in cases of Comparative Example 1 and Comparative Example 2.
In the liquid crystal projector, achievement of the brightness on the screen is an important factor as a display performance. Therefore, in order to achieve the brightness, the reflection type liquid crystal display device having a high aperture ratio and a metal halide lamp being capable of obtaining a high luminance with a small power consumption are mainly used.
The metal halide lamp is a discharge lamp obtained by sealing mercury and a plurality of halogenide metals in a light-emission tube. In the light-emission spectrum of the metal halide lamp, some strong bright lines inherent to the sealed materials are generated. FIG. 4 is a graph showing a spectral distribution of a metal halide lamp used in the liquid crystal projector. As shown in FIG. 4, a metal halide lamp which is most popularly used at present has strong bright lines near 440 nm, 540 nm, and 580 nm in the visible region.
As shown in FIG. 3, when the reflectance of the interface reflected light R2 on the interface between the glass substrate and the liquid crystal ranges from about 3% to about 7%, the interface reflected light R2 has reflection strength which cannot be neglected in a the wavelength of a bright line. For example, when a liquid crystal layer generally having a thickness of about several xcexcm has slight unevenness, interference fringes are generated on the screen by interference between the reflected light IR and the interface reflected light R2, and display quality is degraded.
In particular, the interface reflected light R2 is weak in a transmission type display device because the interface reflected light R2 frequently passes. However, interference becomes great in a reflection type liquid crystal display device because the interface reflected light R2 strongly appears in the reflection type liquid crystal display device, and interference fringes are easily conspicuous.
As described above, interference fringes on the screen generated due to the presence of reflected light from the interface between the liquid crystal layer and the glass substrate conspicuously appear especially in a liquid crystal projector using a reflection type liquid crystal display device and a discharge lamp having a bright line spectrum, so that display quality is degraded.
It is an object of the present invention to provide a liquid crystal display device having an arrangement being capable of suppressing light reflection occurring on the interface between a liquid crystal layer and a glass substrate and a liquid crystal projector having display quality which is improved by using the liquid crystal display device.
It is another object of the present invention to provide a liquid crystal display device which is capable of using a light source having high light efficiency and has high productivity.
To achieve the object described above, from the first aspect of the present invention, a liquid crystal display device according to the present invention comprising:
a pair of substrates, at least one of them being a transparent substrate;
a pair of electrode layers formed on the inner surfaces of the substrates, the electrode layer formed on the inner surface of the transparent substrate being transparent;
a pair of alignment layers formed on the inner surfaces of the substrates, the alignment layer formed on the inner surface of the transparent substrate being transparent; and
a liquid crystal layer intervening between the pair of substrates;
wherein a laminate structure constituted by the transparent electrode layer, the alignment layer, and one or more transparent intermediate layers having a refractive index smaller than that of the transparent electrode layer and larger than that of the liquid crystal layer or the transparent substrate is formed on the inner surface of the transparent substrate.
According to the liquid crystal display device, the laminate structure using the electrode layer and the alignment layer and using a transparent material having an intermediate refractive index between the refractive indexes of these layers as an intermediate layer is formed, so that the thicknesses of the respective layers can be adjusted, reflectances of the interfaces of the layers can be relatively easily reduced, and occurrence of defective display caused by reflection on the interfaces can be prevented.
Preferably, a light source having a bright line in a visible region is used as a light source. The laminate structure is designed to determine, in the bright line wavelength of the light source, the refractive indexes and thicknesses of the layers constituting the laminate structure such that the sum of reflectances, generated on the layer interfaces of the laminate structure, of light propagating from the liquid crystal layer to the transparent substrate is set to be not more than 0.5%.
According to the liquid crystal display device, even if a light source having a bright line which is conspicuously influenced by reflection on the interfaces of the laminate structure formed on the transparent substrate is used, the sum of reflectances generated on the layer interfaces of the laminate structure is set to be not more than 0.5%. For this reason, even if some unevenness of the thickness of the liquid crystal cell is present, occurrence of defective display caused by reflection on the interfaces can be prevented.
Preferably, the other of the pair of substrates comprises, as an electrode layer formed on the inner surface, a reflective electrode layer which reflects light in a visible region.
The laminate structure formed on the inner surface of the transparent substrate is designed such that a first intermediate layer, a transparent electrode layer, and an alignment layer are laminated in order from a transparent substrate side.
From the second aspect of the present invention, there is provided a projection type display apparatus having the liquid crystal display device.
According to the projection type display apparatus, since the liquid crystal display device is used, generation of reflected light which generates interference fringes on a screen to emitted light from the liquid crystal display device used for original display is prevented, and preferable display quality can be maintained.
Since the liquid crystal display device is used, even if a metal halide lamp which has high light efficiency but has a bright line in a visible region is used as a light source, preferable display quality can be maintained.
From the third aspect of the present invention, there is provided a liquid crystal display device using a light source using a bright line in a visible region, comprising:
a pair of opposite substrates;
a liquid crystal sealed between the substrates;
a first transparent material film formed on one of the substrates on which light from the light source is admitted, and being in contact with the liquid crystal; and
a second transparent material film with which the first transparent material film is in contact;
wherein a refractive index of the first transparent material film is set to be an intermediate value between a refractive index of the second transparent material film and a refractive index of the liquid crystal.
Preferably, a reflectance of an interface between the first transparent material film and the liquid crystal is not more than 0.5%.
Preferably, when a wavelength of a bright line generated by the light source or an intermediate value of wavelengths of a plurality of bright lines generated by the light source is represented by xcex, the thickness of the first transparent material film is set to be xcex/4 as an optical film thickness and the thickness of the second transparent material film is set to be xcex/2 as an optical film thickness.